More than 300 hectares of trees will have to be harvested early after a freak combination of hail and fungal damage wiped out about 6 per cent of the ACT's largest commercial pine plantation.

"The severe hail storm [in February] damaged the growing tips of trees and stripped bark and needles from soft tissue on young branches. This damage then allowed Diplodia fungi spores to infect the trees," ACT Forestry coordinator Peter Langdon said.

"This type of fungus affects all species of pine trees and is usually more severe in damaged trees which have previously been stressed by drought, nutrient deficiency or insect damage."

While pine trees are usually harvested at 30 years, Mr Langdon said most of the affected trees were only 15 and 16 years old.

He said there was no viable alternative other than to remove the trees.

"The trees are about 15 years old, so they're about halfway through their commercial life. We've lost that future revenue that we would have got if we'd been able to grow them on," Mr Langdon said.

The damage is concentrated in the Colverwell area of Kowen Forest, near the ridgeway.

In the worst-affected areas, the telltale red-brown colour of infected needles engulfed at least half of each afflicted tree.

Mr Langdon said while the "one-in-35-year" weather event and fungi infection caused up to half a million dollars' worth of damage, the loss will be covered by insurance.

"The ACT's pine plantations are covered by insurance which will negate the loss of harvesting them earlier than normal," Mr Langdon said.

"The timber is still sound so it will be safely used for pulp and sawn timber with no risk of spreading the fungus."

The Kowen pine plantation spans 4700 hectares. Mr Langdon said the rest of the plantation remained viable.

The ACT's other 3000 hectares of pine trees at Pierces Creek and Uriarra were not affected by the storm.

The harvest will finish at the end of August. New trees will be planted from next July.